1. Field of the Invention
The present invention relates to a fuel cell system having a fuel cell which has a supply line and a discharge line for a fuel as well as a supply line and a discharge line for an oxidizing agent. Such a system also has a fuel tank for a fuel comprising hydrocarbons, which is connected to a filter element for preventing the release of hydrocarbons. Furthermore, the invention concerns a method of regenerating the filter element in a fuel cell system.
2. Discussion of the Prior Art
Fuel cells are already known and have gained considerably in significance in recent years, in particular in the area of the automotive industry.
In a way similar to battery systems, fuel cells generate electrical energy by chemical means, the individual reactants being continuously supplied and the reaction product being continuously removed. In this case, the fuel cells are based on the operating principle that electrically neutral molecules or atoms bond with one another and thereby exchange electrons. This process is referred to as the redox process. In a fuel cell, the oxidation and reduction processes are physically separated. The electrons given off during the reduction can be conducted as a current through a load, for example the electric motor of an automobile.
Used as gaseous reaction partners for the fuel cell are, for example, hydrogen as the fuel (anode gas) and oxygen (cathode gas) as the oxidizing agent. If it is desired to operate the fuel cells with a fuel which is readily available and easy to store, such as natural gas or methanol, the hydrocarbon must initially be converted into an oxygen-rich gas by reforming.
When filling a fuel tank with a fuel which comprises hydrocarbons and subsequently converting it into the fuel for the fuel cell, for instance by steam reforming or the like, as well as when venting the fuel tank, hydrocarbons are released in an unwanted manner. Since the hydrocarbons are harmful to the environment and, what is more, can also cause damage in the fuel cell system, they must be bound in a suitable way. This generally takes place by means of a filter element, for example an active carbon filter, as is known for fuel tanks of vehicles with internal combustion engines, for example from WO 97/33765. This filter element must be regenerated from time to time in order to ensure its serviceability. If regeneration does not take place in time, the hydrocarbons can pass through the filter element and into the atmosphere unhindered.
Similarly, hydrocarbons can also get into the atmosphere as a result of gas leaks in the tank system (cf. for example German reference DE 19818697 A).
However, legislation prescribes, for example for Germany in the form of the 21st Bundes-Immissionsschutzverordnung [German federal regulation on air pollution control] of Oct. 7, 1992, that no or only a very small amount of hydrocarbons may get into the atmosphere.
German reference DE 691 26 321 T2 (European reference EP 0559816 B) discloses a power generating system which includes, inter alia, a fuel cell system of the generic type which is provided with devices for supplying and removing fuel and oxidizing agent. Furthermore, this fuel cell system is equipped with a cleaning device for the removal of hydrocarbons. Although the cleaning device is connected to the fuel tank via a line, it does not serve for cleaning the flow of exhaust gas but for cleaning the fuel which is supplied to the fuel cell.
Setting out from this cited prior art, the present invention is therefore based on the object of improving a fuel cell system of the type mentioned above so that the that the disadvantages described are avoided. In particular, it is intended to provide a fuel cell system in which the filter element can be regenerated in a simple and reliable way. Finally, an improved method of regenerating a filter element is also to be provided.
Pursuant to these objects, and others which will become apparent hereafter, one aspect of the present invention resides in a fuel cell system in which a filter element is connected to the discharge line for the oxidizing agent and/or the discharge line for the fuel.
The fuel cell system according to the invention firstly allows the filter element to be regenerated in a simple and low-cost way. For this purpose, the flow of exhaust gas of the oxidizing agent (cathode gas) and/or the fuel (anode gas) is passed through the filter element. As a result, the filter element is regenerated by the flow of exhaust gas passed through it entraining the hydrocarbon molecules. After leaving the filter element, these molecules can then be burned, for example in a device described in more detail further below.
Using the flow of exhaust gas from the fuel cell, which is generally still under superatmospheric pressure when it leaves the fuel cell, for regenerating the filter element, makes it possible to dispense with a pump dedicated to this purpose. In the case of conventional vehicles with an internal combustion engine, the regeneration of the filter element takes place by subatmospheric pressure in a suction pipe. In the case of vehicles with fuel cells, such a subatmospheric pressure is no longer available.
An active carbon filter may be advantageously used as the filter element. Depending on requirements and the application, however, other types of filter are also conceivable, with the result that the invention is not restricted to active carbon filters.
The filter element may be connected to the discharge line for the oxidizing agent or to the discharge line for the fuel or else to both discharge lines. However, it must be ensured that the flow of exhaust gas can entrain the hydrocarbons located in the filter element. In an advantageous embodiment, the filter element is therefore connected to the discharge line for the oxidizing agent, in particular whenever oxygen is used as the oxidizing agent, an adequate amount of which is always present.
Methanol, benzene, methane, natural gas, coal gas, biogas or the like may, for example but not exclusively, be used as the fuel for the fuel cells from which the hydrogen is produced in subsequent processes. Atmospheric oxygen may be used, for example, as a suitable oxidizing agent.
The filter element can be advantageously connected via a connecting line to an arrangement for preparing/generating fuel for the fuel cell from the fuel located in the fuel tank. In this way, once it has flowed through the filter element and bound the hydrocarbons, the flow of exhaust gas from the fuel cell can be further used in the production of the fuelxe2x80x94for example hydrogen.
The filter element is advantageously connected via a connecting line to a burner. In this burner, which may be designed for example as a catalytic burner, the flow of exhaust gas from the fuel cell, which after leaving the filter element also contains hydrocarbons, is burned. The waste heat thereby produced can be used in generating/preparing the fuel, for instance for vaporizing the fuel in a device for gas treatment.
Consequently, passing on the stream of exhaust gas containing hydrocarbons has the additional advantage that the harmful hydrocarbons are burned and that the waste heat produced is not lost but can be utilized. Furthermore, there is no direct connection between the filter element and the atmosphere, with the result that unwanted discharge of hydrocarbons into the atmosphere during operation of the vehicle is prevented.
In a further embodiment, a valve, in particular a shut-off valve, may be provided in the connecting line between the filter element and the burner. Such a shut-off valve is advantageous if a leakage diagnosis is carried out in the fuel tank. In this case, the shut-off valve must be closed. The operating principle of such a leakage diagnosis is discussed in more detail further below.
The fuel tank may be advantageously connected to the filter element via a line, with a valve, in particular a tank protection valve designed as a pressure-relief valve, being provided in the line. The tank protection valve prevents excessive superatmospheric pressure from occurring in the fuel tank.
A valve, in particular a check valve, is preferably provided in the discharge line for the fuel and/or the oxidizing agent connected to the filter element. The check valve protects the fuel cell from hydrocarbons being able to get into the fuel cell from the filter element and damage (poison) the fuel cell.
In a further embodiment, a pressure reducer is provided in the discharge line for the oxidizing agent and/or the fuel connected to the filter element. If the fuel cell system is operated with high pressure, the pressure prevailing in the discharge line is reduced by means of the pressure reducer. In this case, an excess flow of exhaust gas can be supplied, for example via an appropriate line, to the arrangement for generating/preparing fuel, in particular the burner. The pressure reducer is advantageously designed to be closable, for example electrically closable. In particular if a leakage diagnosis, described in more detail further below, is to be carried out in the fuel tank, it must be possible for the pressure reducer to be closed.
The discharge line for the fuel and/or the discharge line for the oxidizing agent may be advantageously connected to the arrangement for generating/preparing fuel, in particular to the burner.
In a further embodiment, at least one pressure sensor is provided in the fuel tank. The pressure sensor can be used, for example in the leakage diagnosis in the fuel tank, for measuring a pressure drop taking place in the fuel tank.
According to a second aspect of the invention, a method of regenerating a filter element, in particular an active carbon filter, is provided. The filter element is provided in a fuel cell system according to the invention, as described above, and the filter element is connected in terms of lines to a fuel tank. The method is defined according to the invention wherein the flow of exhaust gas of the oxidizing agent and/or the fuel from the fuel cell is introduced into the filter element.
This provides a simple and low-cost possible way of regenerating the filter element with the advantages described above. For the advantages, consequences, effects and operating principle of the method according to the invention, reference is made to the full content of the comments made above with respect to the fuel cell system according to the invention.
The flow of exhaust gas of the oxidizing agent and/or the fuel passed through the filter element may subsequently be introduced into a device for generating/preparing fuel, in particular into a burner. In this way, the flow of exhaust gas for regenerating the filter element is initially passed through the filter element and subsequently burned together with the desorbed hydrocarbons from the filter element, for example in the burner. The harmful hydrocarbons are burned, utilizing the waste heat produced. Release of the hydrocarbons from the filter element into the environment is prevented.
In a further embodiment, the pressure of the flow of exhaust gas introduced into the filter element can, if need be, be specifically set by means of a pressure reducer, which leads to the advantages described further above.
The invention advantageously makes it possible to carry out a leakage diagnosis in a fuel tank of a fuel cell system according to the invention, as described above, the fuel tank being connected to a filter element. In this case, it is provided that a pressure reducer fitted in the discharge line for the oxidizing agent and/or the fuel from the fuel cell, connected in terms of lines to the filter element, is closed. The fuel tank is subjected to a certain superatmospheric pressure and the pressure drop in the fuel tank is measured by means of a pressure sensor.
The basic idea of this method is that, for testing the tightness of the fuel tank, the tank is subjected to a certain pressure. Since the pressure reducer is closed, the pressure cannot escape if the fuel tank is in the proper state. A pressure drop in the fuel tank can accordingly occur only in the presence of a leak. The possible pressure drop is measured by means of the pressure sensor in the fuel tank. The leakage diagnosis is advantageously carried out during the warm-running phase of the fuel cell system. If the filter element is connected via a connecting line to an arrangement for preparing/generating fuel, in particular a burner, a valve, in particular a shut-off valve, is fitted in the connecting line and has to be closed in order that the pressure built up in the fuel tank cannot escape via the filter element.
In a further embodiment, the fuel cell system according to the invention may be used in particular for operating a vehicle, in particular for electrical propulsive drive. Similarly, the method according to the invention may preferably be used for a fuel cell system for operating such a vehicle.
Fuel cell technology in the vehicle sector represents the preferred field of use of the invention. Nevertheless, other possible uses are also conceivable. To be mentioned here for example are fuel cell systems for mobile equipment through to power generating installations in which the hydrogen is produced from methanol, benzene or the like. The fuel cell technology is also suitable for the decentralized supplying of energy to households, industrial installations or the like.
The present invention is not restricted to particular types of fuel cells, with the result that the invention can be used in conjunction with all types of fuel cells. Such fuel cells are, for example, alkaline fuel cells (AFC), fuel cells with a polymer membrane (PEMFC or DMFC=direct methanol fuel cell), phosphoric acid fuel cells (PAFC), molten carbonate fuel cells (MCFC), solid oxide fuel cells (SOFC), or the like. The different types of fuel cell operate with different levels of pressure and temperature.
In a particularly preferred way, the present invention may, however, be used in conjunction with fuel cells with polymer membranes (PEM or DMFC). These fuel cells have a high electrical efficiency, cause only minimal emissions, have an optimum part-load behavior and are essentially free from mechanical wear.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of the disclosure. For a better understanding of the invention, its operating advantages, and specific objects attained by its use, reference should be had to the drawing and descriptive matter in which there are illustrated and described preferred embodiments of the invention.